Wellbores in the oil and gas industry are generally drilled by rotating a drill bit conveyed into the wellbore as attached to a drill string. A bottom hole assembly (BHA) is positioned near the end of the drill string and includes the drill bit. The drill string can include multiple lengths of drill pipe or tubing, or may alternatively comprise coiled tubing. In some cases, the drilling assembly includes a drilling motor or a “mud motor” that rotates the drill bit. In other cases, the drill bit may be rotated by rotating the entire drill string from a surface drilling rig.
During drilling, a drilling fluid or “mud” is supplied, often pumped under pressure, from a source at the surface into the drill string. When a drilling motor is used, the drilling fluid drives the drilling motor and then discharges at the bottom of the drill bit. The drilling fluid returns uphole via the annulus defined between the drill string and the wellbore and carries with it cuttings and debris generated by the drill bit while drilling the wellbore.
At various times while drilling or completing a wellbore, the drilling fluid may be reverse circulated through the wellbore in an attempt to clean out the wellbore. For example, reverse circulation is commonly employed for sand cleanout purposes following wellbore fracturing or hydrajetting operations. In reverse circulation, a surface pump used to circulate the drilling fluid through the drill string and into the surrounding annulus (i.e., forward circulation), is instead used to pump the drilling fluid first into the annulus and then into the drill string at a location at or near the bottom of the drill string. The return fluid flows up the drill string, carrying with it sand, debris, and drill cuttings.
Reverse circulation forces the drilling fluid to flow through the relatively smaller inner diameter of the drill string in returning to the surface as opposed to the larger annulus, and thus achieves better fluid velocity. The increased fluid velocity enhances the debris (sand) suspension capabilities of the drilling fluid as compared to direct (i.e., forward) circulation. More particularly, greater fluid velocity helps entrain and lift the debris more efficiently, which increases the overall cleaning efficiency or effectiveness of the operation for the well. This is true, however, only if the debris is suspended and loose within the wellbore. If the debris is consolidated and settled, reverse circulation may lose this advantage due to an inability to agitate the consolidated debris. While increasing the pressure differential of the reverse circulation may agitate some of the consolidated debris to be circulated out, such increased pressures may also result in damage to the drill string (coiled tubing) or in fluid losses into the subterranean formations surrounding the wellbore.